Color printers have become exceedingly popular. Previously, such printers were mainly used only for professional purposes, since their cost could run into the thousands of dollars. Professional artists and entities concerned with printing color images and documents on various types of media had at their disposal high-end printers that could generate very life-like color prints. More recently, however, the cost of color printers, including laser printers but more usually inkjet printers, has plummeted, resulting in their purchase by home users and other non-professionals. With the advent of applications like digital photography, such low-cost color printers are increasingly being used to print color prints of photos, computer-drawn images, and other types of documents.
Regardless of the type of color printer, color calibration is an important process for color printers and other types of devices. Color calibration generally is the matching of colors to a base color, such as a Pantone color, from one device to another. For example, matching the color one sees on a computer display to that of printer output is within the realm of color calibration. Calibration maintains color consistency from print job to print job, from one day to the next, and so on. Calibration is especially important when maintaining critical colors, such as colors in company logos, production of multiple prints in a single print job, production of various jobs, and so on. Modem color printers can produce excellent color output, but colors tend to drift over time, causing consistency and other problems for users.
A color calibration routine can contain a test target of a number of color patches that range from 0 to 100% ink coverage for each ink color the printer uses. The purpose of this test target is to allow users to evaluate ink coverage. After printing the calibration target, users typically read the target with a device that generates density or other types of values, which are entered into the calibration routine to determine the adjustment necessary to provide for matched colors. Density values in particular provide a numeric value that correlates to ink film thickness, where high density values relate to heavier ink coverage, and lower density values relate to lighter ink coverage. Devices that can measure such density and other values include colorimeters, spectrophotometers, and other types of devices. A common type of color calibration routine is referred to as linearization, in which a straight-line relationship between color input and color output is determined.
A difficulty with color calibration is that the measurements taken by the color calibration devices, referred to herein generally as color values, can exhibit a number of different problems. For example, it is difficult to confine the measurement region to just the color patch in front of the sensor of the device. Even if 99% of the light reaching the sensor is from the patch of interest, this still means that 1% comes from other areas. Thus, neighboring color patches can affect the measured value of the patch under observation if they are significantly different in color.
Furthermore, the sensors of the color calibration devices are typically sensitive to variations in height. Light targeted on the media by the sensors will move into and out of the field of view of the sensor with a change in height. Any kind of bow in the platen or any non-parallel motion of the carriage, where the device is attached to the carriage, will result in skewed values. Ideally the only light component captured by the sensor is due to the light targeted on the media by the sensor. More practically, however, the color calibration device operates in an area where a non-trivial amount of other, ambient light is detected by the sensor. This ambient light artificially adds a component to the color values, corrupting them.
Finally, effects such as thermal changes, current sink setting inaccuracies, and so on, can impact the values read by the sensors. These effects are usually slow enough that they do not affect a single scan, but rather affect scans performed over time. The result is that patches read by the same sensors of the same devices may generate different color values from one hour to the next, and from one day to the next, causing inconsistent color calibration. For these and other reasons, therefore, there is a need for the present invention.